911 DME pressure compensation?

[Bart_dood]

Did a search but it came up zinch, does anyone know if the Bosch DME injection system fitted to my 88 carrera compensates for changes in air pressure?

If not I guess these cars run rich at altitude and leaner on high air pressure days?

Perhaps the fact the air flow sensor is a spring flap it might compensate somewhat as lower density air might not force the flap open as wide?

[burgermeister]

There is some sort of compensation switch under your seat next to the DME. I think it's a simple on / off at 4000 feet or something of that nature. Not sure what exactly it does to the DME.

But, I would expect the DME door to be at least somewhat of a mass airflow sensor. I'd think the denser the gas, the further the door opens, especially at higher flow rates when the door is open. Or so it seems to me.

[ischmitz]

The "barn door" flapper air flow sensors on the 3.2 and 964 measures air volume and require adjustment for barometric pressure. The air mass is a function of air volume and ambient pressure. There is a barometric pessure sensor under the drivers seat that is feeding barometric pressure information to the DME. It is an adjustable resistor network that is read by one chanel of the multiplexed A/D converter. The readout is used to adjust the mixture (fuel quanttiy) based on barometric pressure.

The (MAF) hot wire air flow sensors on the 993 and later measure air mass and do not require this compensation.

Ingo

[stlrj]

Originally Posted by burgermeister

There is some sort of compensation switch under your seat next to the DME. I think it's a simple on / off at 4000 feet or something of that nature. Not sure what exactly it does to the DME.

But, I would expect the DME door to be at least somewhat of a mass airflow sensor. I'd think the denser the gas, the further the door opens, especially at higher flow rates when the door is open. Or so it seems to me.

The altitude switch weakens the mixture at 2600' above sea level per Spec Book.

[911st]

Can the altitude switch be disconected if one converets to a MAF sensor system?